The present invention relates to measuring characteristics of a transmitter, and more particularly, to a gain asymmetry characterizing circuit capable of determining characteristics of gain asymmetry possessed by a transmitter, and a method thereof.
Radio frequency (RF) circuits are commonly applied in transceivers, transmitters and receivers, to provide mixed-signal systems on a chip for wireless applications. When an RF transmitter utilizes a passive mixer without reverse isolation, an asymmetry phenomenon will occur in the RF transmitter.
Please refer to FIG. 1, which is a diagram illustrating a traditional radio-frequency (RF) transmitter. As shown in FIG. 1, the RF transmitter 100 includes an in-phase input end TXI and a quadrature input end TXQ. For example, the in-phase input end TXI is used to generate the in-phase component of a sinusoid, and the quadrature phase input end TXQ is used to generate the quadrature phase component of a sinusoid. There is a 90 degree phase difference between the in-phase and quadrature phase components. By adjusting these two components, a goal of adjusting the amplitude, frequency and phase of the sinusoid can be achieved. The in-phase transmission path includes a filter 20, a resistor-capacitor (RC) circuit 30, a passive mixer 40 and an amplifier (e.g. a programmable gain amplifier) 50, wherein the filter 20 includes a transfer function. Similarly, The quadrature phase transmission path includes a filter 22, a RC circuit 32 and a passive mixer 42. These two paths are joint at the output end of the passive mixer 40. The amplifier 50 outputs signals to an output TXO. The signals at the input ends of the passive mixers 40 and 42 will be affected by the signals at the output ends of the passive mixers 40 and 42, respectively, resulting in the asymmetry transfer function in RF. For example, an example of an asymmetry function is shown in FIG. 5. Although the asymmetry can be compensated in the digital domain to achieve spectrum flatness, the transfer function has to be characterized in advance to achieve the desired compensation. Measuring the gain asymmetry for an RF transmitter requires the RF transmitter to have certain characteristics such as good linearity, low noise, good in-phase quadrature (I-Q) matching and a flat frequency response over process, voltage and temperature (PVT) variations. Unfortunately, the RF frequency response can be easily corrupted by external loading from the output TXO. That is, It is difficult to use RF circuits to measure gain asymmetry, because RF circuit frequency response is sensitive to linearity, PVT, external loading and etc.
Therefore, there is a need for a novel mechanism for characterizing the gain asymmetry of an RF transmitter that can achieve spectrum flatness.